The present invention relates to the field of signal generation through vibration of a large mass. More particularly, the invention relates to a seismic exploration device for storing large quantities of energy and for selectively discharging such energy to vibrate a large mass.
Seismic exploration methods indicate the physical properties and spatial disposition of geologic formations underlying the earth surface. In land based seismic systems, mechanical vibrators or explosive charges initiate a pressure wave in earth materials. In marine systems, a seismic source array comprising air guns or other pressure source wave generators are towed by a vessel through the water. Source waves from a seismic source travel downwardly through the subsurface formations, and portions of the seismic wave energy are reflected, refracted and transmitted by geologic interfaces between subsurface geologic units. The returning reflected waves are detected by a sensor or sensor arrays located at a known position relative to the energy source.
Seismic sensors in land based or marine systems comprise transducers which convert pressure, velocity and acceleration into electric signals. The output signals are recorded and processed to indicate mechanical, acoustic and structure characteristics of geologic units. For example, the propagation time of a seismic wave reflected from a point is proportional to the depth of the reflection point. Travel times from multiple, spatially diverse source and receiver pairs facilitate construction of maps representing the spatial disposition of subsurface geologic units.
Seismic source energy can be provided by explosive charges such as dynamite or other explosive compounds. In U.S. Pat. No. 4,324,310 to Wener et al. (1982), a high resolution seismic data acquisition system used an electronically fired shotgun source. One disadvantage of explosive charges are that they are typically self destructive and cannot be used multiple times. Explosive charges also present a multitude of handling and storage problems, can destroy natural and anthropogenic structures, and can destroy plant and animal life.
Conventional land based seismic vibrators transmit vibration energy to the earth land mass over a period of time. Seismic vibrators are typically driven by an oscillating hydraulic piston engaged to a stiff plate in contact with the ground. The hydraulic cylinder is also engaged with a substantial reaction mass. When hydraulic pressure is applied to the piston within the cylinder, opposing forces are applied to the reaction mass and the baseplate. The force applied to the baseplate is transmitted into the earth as pressure waves or shear waves into the earth material contacted. The duration and strength of waves introduced into the earth depends upon the amount of energy available to drive the pump, capacity of the hydraulic pump, characteristics of the storage device, and mechanical efficiency of the system. The hydraulic cylinder is powered by a hydraulic pump typically driven by an internal combustion engine. An electronically actuated valve system controls the flow of hydraulic fluid to the hydraulic cylinder and is capable of varying the pressure as a function of time. It is also possible to store hydraulic energy by compressing a gas with hydraulic oil in an accumulator or other pressure vessel.
Vibratory seismic sources impart energy into the land mass and water over a period of time at typically lower energy levels than explosive systems. A variety of mechanisms have been employed to produce vibratory seismic source signals. U.S. Pat. No. 4,692,672 to Okuno (1987) disclosed a vibration wave motor which applied a periodic voltage to an electro-restrictive element. The speed of the movable member was reduced without reducing the amplitude of the periodic voltage, a synchronizing pulse generator produced a rectangular pulse, and this pulse was subsequently modulated and amplified.
U.S. Pat. No. 4,734,894 to Cannelli et al. (1988) disclosed a seismic source having an electro-acoustic transducer connected to a set of capacitors. A high voltage power supply provided power to the capacitors, and the discharge produced an acoustic pulse through a transducer in a liquid medium to generate seismic waves.
U.S. Pat. Nos. 4,850,449 to Cheung (1989) and 5,005,665 to Cheung (1991) disclosed a seismic vibrator driven with piezoelectric transducers to produce improved signal resolution at higher frequencies than hydraulic or pneumatic vibrators. The piezoelectric vibrators could be used at the earth surface, within an open borehole, or when buried below the earth surface. A conventional power source such as an electric generator supplied power which was then amplified and provided to piezoelectric vibrators within a baseplate.
Conventional seismic sources are limited by the power sources and the deliverability of such power to the target mass. Conventional capacitor banks provide relatively low storage density for certain power requirements. Accordingly, a need exists for an improved apparatus for storing and discharging large amounts of energy suitable for use as a seismic energy source. The apparatus should be portable, should be capable of generating large quantities of seismic source energy, and should be capable of generating a signal which can be modified to address different conditions and applications.